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We present a micro-integrated, extended cavity diode laser module for space-based experiments on potassium Bose-Einstein condensates and atom interferometry. The module emits at the wavelength of the potassium D2-line at 766.7 nm and provides 27.5 GHz of continuous tunability. It features sub-100 kHz short term (100 μs) emission linewidth. To qualify the extended cavity diode laser module for quantum optics experiments in space, vibration tests (8.1 gRMS and 21.4 gRMS) and mechanical shock tests (1500 g) were carried out. No degradation of the electro-optical performance was observed.
© 2014 Optical Society of America
1. Introduction
Tunable diode lasers are essential tools in different fields like coherent optical communication or fundamental physics, for which quantum sensors based on cold atomic samples play an increasing role. Especially quantum sensors like cold atom microwave atomic clocks, optical atomic clocks [1–3] and atom interferometers [4–10] are leaving optical labs. Today precision quantum optics applications in space or in a micro-gravity environment [11] require not only a laser system with good electro-optical performance such as an excellent short-term frequency stability of a few kHz or a wide continuous frequency tuning range but also a very compact design which provides reliable mechanical and thermal stability. Extended cavity diode lasers (ECDLs) [12–15] are very well suited for different fields of applications for their excellent spectral stability, however they typically lag mechanical stability and reliability because of their complex mechanical structure. This issue has recently been resolved by the realization of a micro-integrated rubidium ECDL (Rb-ECDL) [16] for precision quantum optics experiments in space. Numata et al. realized a planar-waveguide extended cavity diode laser [17] at the wavelength of 1540 nm by a micro-integration approach for optical sensing applications.
In this paper we report on a new generation of our micro-integrated ECDL. The micro-integrated ECDLs are used for experiments on potassium Bose-Einstein condensates (BECs) and for atom interferometry onboard a drop capsule at the ZARM drop tower, Bremen, Germany. Furthermore, the ECDLs will be used for demonstration of atomic spectroscopy on potassium within the frame work of sounding rocket missions funded by the German space agency DLR in 2015.
The micro-integrated potassium ECDL (K-ECDL) presented here possesses an extended cavity length of approximately 30 mm and provides an intrinsic linewidth [16] of 3 kHz and an optical power in excess of 35 mW behind a micro-optical isolator with an isolation of 30 dB. The micro-integrated ECDL features a short term full-width-half-maximum (FWHM) linewidth of significantly less than 100 kHz. To qualify the micro-integrated ECDLs for future quantum optics precision experiments in space, vibration tests (8.1 gRMS and 21.4 gRMS) and mechanical shock tests (1500 g) were carried out.
The paper is organized as follows. In Section I we describe the design of the micro-integrated ECDL. In Section II we experimentally investigate the electro-optical performance and the short-term frequency stability of a micro-integrated K-ECDL. Section III describes the result of the mechanical stress tests carried out on an ECDL. To this end, we compare relevant electro-optical parameters of the laser module before and after the stress test. Section IV summarizes the results.
2. Micro-integration concept
The micro-integrated ECDL consists of a 1 mm long, double quantum well AlGaAs-based ridge-waveguide (RW) gain chip featuring two anode sections, collimating aspheric micro-lenses, a volume holographic Bragg grating (VHBG), a micro-optical isolator, a micro-thermoelectric cooler (μ-TEC), micro-temperature sensors, and an electronic interface. All electronic components are either soldered or adhesively bonded with space compatible adhesives on an aluminum nitride (AlN) ceramic base plate with a footprint of only 80×25 mm2. The module takes up a volume of 30 cm3 and only weighs 40 grams. Figure 1 shows a photograph of the micro-integrated ECDL module. The front facet of the laser chip and the VHBG define the approximately 30 mm long extended cavity. The laser chip’s rear output is collimated by an aspheric lens with 2 mm focal length towards a VHBG in Littrow configuration which provides selective optical feedback to enforce single-mode operation. The VHBG provides a nominal resonant diffraction efficiency of 70 % at the potassium wavelength (766.7 nm) and a nominal FWHM bandwidth of 22 GHz. The VHBG is thermally stabilized by means of a μ-TEC and a micro-temperature sensor to control the emission frequency of the laser module. The laser chip’s front output is collimated by an identical lens and is then passed through a micro-optical isolator providing 30 dB of isolation at the cost of an insertion loss of 3 dB. The electronic interface provides access to temperature sensors located close to the VHBG and to a sensor that determines the temperature of the AlN ceramic body. The interface also provides access to the μ-TEC that is used to control the temperature of the VHBG. The electronic interface further includes a transistor in parallel to the laser chip as well as a bias Tee for fast injection current control and modulation.
Fig. 1 Photograph of the micro-integrated ECDL module. The laser chip, collimating lenses, a micro-optical isolator, an electronic interface, micro-temperature sensors, and a micro-thermoelectric cooler (μ-TEC) that carries the volume holographic Bragg grating (VHBG) are all micro-integrated on the aluminum nitride ceramic plate.
The hybrid integration of all passive optical components is carried out by actively aligning the lenses and the VHBG with precision manipulators. Once the optimal alignment is achieved, the components are glued in place with UV curing, space compatible adhesives. During alignment and UV curing the position of the lenses is typically controlled with a spatial resolution of better than 1 μm and an angular resolution of better than 1 mrad.
3. Electro-optical characterization of the micro-integrated ECDL
In this section we describe the electro-optical properties such as output power, optical spectrum and frequency tunability, and spectral stability of the micro-integrated K-ECDL.
At first, the dependence of the output power on the injection current (PI-characteristics) is determined at a micro-optical bench (MIOB) temperature of 29.0°C and is shown in Fig. 2(a). In this measurement the injection current is synchronously tuned at 17 mA/K with the VHBG temperature which is increased from 24.3°C to 32.3°C. This synchronization allows to eliminate mode hops between neighboring ECDL modes that are observed when the injection current is tuned [16]. The threshold is reached at an injection current of 116.6 mA. The laser shows a discontinuous turn-on behavior which is attributed (i) to the interaction between different longitudinal modes of the laser and (ii) to intensity-dependent losses introduced by parts of the gain chip that are not pumped, e.g. near the facets or between the two sections of the gain chip. The ECDL provides an output power of 35 mW behind the micro-optical isolator at an injection current of 250 mA. For the injection current range from 110 mA to 250 mA no thermal roll-over is observed. However, a small modulation of the output power is recognized when the injection current is tuned. This is attributed to mode competition between neighboring longitudinal modes [16]. Figure 2(b) shows how the optical frequency tunes with the injection current. Synchronization of the VHBG temperature to the injection current provides a continuous tuning range of 27.5 GHz around the potassium D2-line at 766.7 nm, which is met at an injection current of 179.5 mA and at VHBG and MIOB temperatures of 28.0°C and 29.0°C, respectively.
Fig. 2 (a) Optical power of a K-ECDL vs. injection current. The VHBG temperature is synchronized to the injection current to avoid mode hops between adjacent ECDL modes. (b) Optical frequency (left axis) of a K-ECDL vs. injection current. The right axis shows the VHBG temperature. In both cases the MIOB temperature is stabilized to 29.0°C.
We next determine the optical emission spectrum of the K-ECDL. Figure 3 depicts the optical spectrum as a function of the injection current at temperatures of 29.0°C and 28.0°C for the MIOB and for the VHBG, respectively. The optical spectra are recorded with an Advantest Q8384 optical spectrum analyzer with a resolution bandwidth of 10 pm FWHM. Single mode operation is maintained during the full current sweep. Mode hopes are clearly visible and occur whenever the ECDL modes have been tuned by more than one free spectral range of the ECDL. However, the laser always locks to the mode that is closest to the VHBG center frequency. In order to determine the side mode suppression ratio (SMSR), spectra are sampled at the injection currents of 130 mA (9.6 mW) and 240 mA (33.4 mW) and are shown in Fig. 4. The ECDL provides a SMSR of at least 45 dB (at 100 pm offset from the carrier) at both injection current settings.
Fig. 3 Optical spectra of a K-ECDLvs. the injection current. The MIOB and VHBG temperatures are stabilized to 29.0°C and 28.0°C, respectively.
Fig. 4 Optical spectra of a K-ECDL recorded at injection current settings of 130 mA (9.6 mW) and 240 mA (33.4 mW). The SMSR corresponds to at least 45.6 dB. The MIOB and VHBG temperatures are stabilized to 29.0°C and 28.0°C, respectively.
We now determine the short term linewidth of the laser by analyzing the beat note between two nominally identical micro-integrated ECDLs. Two diode laser controllers (ILX Lightwave LDC-3724C) with low-pass filters (ILX Lightwave LND-320) drive the injection currents of two lasers and stabilize the two VHBG temperatures by means of the μ-TECs. Two temperature controllers (ILX Lightwave LDT-5525C) stabilize the MIOB temperature of the ECDLs. The output beams are fiber-coupled and superimposed on a fast photodetector (New Focus 1554-B) by means of a fiber splitter. A RF spectrum analyzer (Rohde and Schwarz FSW26) records the photodetector signal. During the linewidth measurements the diode laser injection currents of both ECDLs are set to the same values for each individual linewidth measurement. The MIOB and VHBG temperatures of both ECDLs are stabilized to 29.0°C and 28.0°C, respectively.
In order to determine the FWHM and the intrinsic [16] linewidths we measure the RF beat note signal of the free running K-ECDLs. For this, we use the IQ (in-phase/quadrature) analyzer of the RF spectrum analyzer (R & S FSW26) to record time domain data [18]. From the time domain data we derive the RF beat note spectrum and the single-sided power spectral density (PSD) of the frequency noise (Sν(f)) of the RF beat note signal. To this end the recorded data set is divided into intervals of equal length, each of which is then analyzed individually. The resulting individual RF spectra are frequency-shifted such that the carrier center frequencies coincide. Both, the individual frequency noise spectra and the individual RF spectra are then averaged. The advantage of this approach over more common approaches is that the beat note measurements do not require relative frequency stabilization. Details of this method are described elsewhere [18].
Two RF beat note spectra for injection current settings of 130 mA and 240 mA are illustrated in Fig. 5(a). For this analysis a data set of 100 ms was recorded with an IQ bandwidth of 160 MHz. The data set was then divided into intervals of 100 μs, which corresponds to a resolution bandwidth of 10 kHz. The FWHM beat note linewidth is determined by means of a Gaussian fit and divided by the factor of 21/2 to obtain the FWHM linewidth of an individual laser as the beat note spectrum is derived from two nominally identical ECDLs [19]. The result of the FWHM linewidth measurements for various settings of the output power is shown in Fig. 5(c). A minimum FWHM linewidth of 47 kHz is obtained at an output power of 9.6 mW (130 mA), which is the lowest power for which linewidth measurements were carried out. With increasing injection current technical noise, which is considered to be dominated by current driver noise, is deteriorating the spectral stability of the laser at small Fourier frequencies. The resulting frequency noise PSDs for the injection current settings of 130 mA and 240 mA are shown in Fig. 5(b). Here the same data set as used before was divided into intervals of 10 ms, which allows for a frequency resolution of 100 Hz. The technical noise adding in at Fourier frequencies below 1 MHz is clearly visible. Above 1 MHz the frequency noise PSD is white. The intrinsic linewidths calculated from the white noise floor [16] for various settings of the output power is depicted in Fig. 5(c). The intrinsic linewidth decreases with a 1/Pout dependence. At an output power of 9.6 mW the white noise floor corresponds to 5×103 Hz2/Hz (Fig. 5(b)), from which an intrinsic linewidth (FWHM) of 8 kHz is obtained as shown in Fig. 5(c). At an output power of 33.4 mW we find an intrinsic linewidth as small as 3 kHz, which is obtained from the white noise floor of 2×103 Hz2/Hz.
Fig. 5 (a) RF beat note spectra with a resolution bandwidth of 10 kHz for two injection current settings of 130 mA (9.6 mW) and 240 mA (33.4 mW). Both spectra are individually normalized to the corresponding peak power spectral density. (b) Single-sided frequency noise power spectral density (Sν (f)) for the same injection current settings. (c) FWHM and intrinsic linewidth for various values of the output power of the K-ECDL. All curves are derived from heterodyne linewidth measurements between two nominally identical, free running K-ECDLs. During the linewidth measurements, the MIOB and VHBG temperatures are stabilized to 29.0°C and 28.0°C, respectively.
This performance has to be compared to the performance of other state-of-the art ECDLs that were developed for spectroscopy of ultra-cold alkali atoms. For example, Baillard and coworkers [20] developed an ECDL for a Cs atomic clock in space (PHARAO) [21]. They used an intra-cavity interference filter as a frequency selective element and achieved an output power of a few 10 mW with a short term and intrinsic linewidth of 157 kHz and 14 kHz, respectively. Gilowski and co-workers [22] applied this concept to realize lasers for experiments on ultra-cold Rb atoms and used a tapered gain chip to boost the output power of the laser. They achieved about 1 W of optical power at 780 nm with a short term and intrinsic linewidth of 187 kHz and 85 kHz, respectively. The ECDL presented here provides some improvement in terms of linewidth (short term linewidth as small as 47 kHz, intrinsic linewidth as small as 3 kHz) and features a mass and a form factor significantly smaller than those cited above.
4. Space relevant mechanical stress tests
Finally we report on the mechanical stability of our micro-integrated ECDL by a comparison of the electro-optical characteristics before and after mechanical stress tests. For these tests we used the ECDLs that were configured for spectroscopy of the rubidium D2-line at 780.2 nm (Rb-ECDL). Their technology and physical realization is identical to that of the ECDLs for potassium spectroscopy with the only exception, that the VHBGs feature different Bragg wavelengths. To qualify the resilience against mechanical stress, random vibration tests at two different stress levels (8.1 gRMS [23] and 21.4 gRMS [24]) were carried out. To this end, the laser modules were mounted on a test platform to which random vibrations with a well-defined amplitude spectrum were applied for 2 minutes. This stress test was carried out for all threes axes consecutively. Following the random vibration tests mechanical shock tests [24] were carried out along all three axes consecutively, each with a peak acceleration of 1500 g.
The optical frequency and the output power, specifically the laser threshold are very sensitive electro-optical properties of a laser. Therefore we compare the optical frequency and the output power as a function of the injection current before and after the mechanical stress tests. In both cases the MIOB and VHBG temperatures are stabilized to 29.0°C and 28.0°C, respectively. Before and after the stress tests, as shown in Fig. 6(a), the laser threshold corresponds to 54 mA and the slope efficiency is 0.40 W/A behind the optical isolator. Before any mechanical stress test the wavelength of the Rb D2-line (780.24 nm) is reached in single-mode operation at an injection current of 133.5 mA, corresponding to an output power of 26.6 mW. After the stress tests the same wavelength is reached at an injection current of 136.25 mA with an output power of 26.8 mW. Figure 6(b) shows the tuning of the emission frequency with injection current before and after the stress tests. After the stress tests, at the injection current settings between 107 mA and 150 mA mode hops by two free spectral ranges occur. This is attributed to the influence of mode competition between different longitudinal modes. The interplay between the different ECDL longitudinal modes critically depends on the exact value of the system parameters like chip temperature and ECDL longitudinal mode frequencies. Because experimental conditions cannot be repeated exactly after the stress tests it is expected that location and size of the mode hops will not exactly repeat. Nevertheless, frequency tuning before and after stress tests does not only repeat qualitatively but also rather quantitatively. The experimental result shows that the micro-integrated ECDL module can be reliably tuned without any mode-hop within one free spectral range also after the stress test. In conclusion, the electro-optical performance of the module was not affected by the mechanical stress tests.
Fig. 6 (a) The output power of the Rb-ECDL as a function of the injection current before and after the mechanical stress tests (vibration tests with 8.1 gRMS and 21.4 gRMS, mechanical shock test with 1500 g). (b) The optical frequency as a function of the injection current before and after the mechanical stress tests (vibration tests with 8.1 gRMS and 21.4 gRMS, mechanical shock test with 1500 g). In both cases the MIOB and VHBG temperatures are stabilized to 29.0°C and 28.0°C, respectively.
5. Conclusion
We have demonstrated a micro-integrated, narrow linewidth ECDL for precision spectroscopy of ultra-cold potassium ensembles (766.7 nm). The micro-integrated ECDL consists of a 1 mm long, double quantum well AlGaAs based ridge-waveguide laser chip, collimating aspheric micro-lenses, a volume holographic Bragg grating, a micro-optical isolator, a micro-thermoelectric cooler to stabilize the VHBG, micro-temperature sensors, and an electronic interface. All parts are either soldered or adhesively bonded with space compatible technologies on an aluminum nitride ceramic base plate with a footprint of only 80×25 mm2 and a weight of 40 grams. The laser provides an excellent mechanical stability and reliability under mechanical stress tests (vibration tests with 8.1 gRMS and 21.4 gRMS, mechanical shock tests with 1500 g). This was proven by the comparison of its electro-optical performance before and after the stress test which revealed no degradation. With an extended cavity length of approximately 30 mm the micro-integrated, K-ECDL provides a short term FWHM linewidth of 47 kHz and an intrinsic linewidth as small as 3 kHz. The K-ECDL provides at least 27.5 GHz of continuous tunability.
Acknowledgments
This work is supported by the German Space Agency DLR with funds provided by the Federal Ministry of Economics and Technology (BMWi) under grant number 50WM1240 and 50WM1132, and by the Future and Emerging Technologies (FET) programme within the Seventh Framework programme for Research of the European Commission, under FET Open grant number 250072.
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23. Qualification level tests within the framework of the TEXUS sounding rocket program of the German Space Agency DLR, amplitude spectrum 20 ... 2000 Hz.
24. Stress level required for specific low-earth-orbit applications.
